Previous behavioral studies have shown that instructions about stimulus-response (S-R) mappings can influence task performance even when these instructions are irrelevant for the current task. In the present study, we tested whether automatic effects of S-R instructions occur because the instructed stimuli automatically activate their corresponding responses. We registered the lateralized readiness potentials (LRPs) that were evoked by the instructed stimuli while participants were performing a task for which those mappings were irrelevant. Instructed S-R mappings clearly affected task performance in electrophysiological and behavioral measures. The LRP was found to deflect in the direction of the response tendency that corresponded with the instructed S-R mapping. Early activation of the instructed response was observed but occurred predominantly on slow trials. In contrast, response conflict evoked by instructed S-R mappings did not modulate the N2 amplitude. The results strongly suggest that, like experienced S-R mappings, instructed S-R mappings can lead to automatic response activation, but possibly via a different route.